Sometimes when operating CW I want to send numbers that are negative. In particular when operating outdoors during the winter and someone asks the temperature, I would like to come back in CW with -5C. For some events like Freeze Your Butt Off, the temperature is a required part of the exchange. I realize that almost always the temperature in Fahrenheit will be above zero, but as a Canadian Ham I would prefer to use Celsius.
The best I have come up with so far is to send M5C, but I am not sure that this will be well understood.